The ignition system for internal-combustion engines including motorcar engines causes an engine ignition through an inductive load, such as a transformer. The transformer is driven by a switching element of a semiconductor device. The switching element is operated by a drive circuit on the basis of a firing signal output from a computer for controlling the engine.
The semiconductor device employed in the ignition system is equipped with protection functions (see, for example, Patent Documents 1 to 4). For example, the semiconductor device is equipped with the function of interrupting the load current in order to avoid a breakdown caused by burnout in a case where the ON signal is continuously applied for a predetermined time or more. Under ordinary circumstances, the operation of interrupting the load current, which is the interruption operation performed by the semiconductor device in order to protect itself, is highly likely to be performed at a timing different from that for outputting the firing signal from the computer for controlling the engine.
The problems including an engine backfire or an engine knocking may occur depending on the timing of the interruption operation. As a result, the machine mechanism of the engine could be broken in the worst case. In order to prevent these problems, the semiconductor device is equipped with the function of providing the slow interruption of load current and preventing unnecessary ignition operation. The semiconductor device further includes provisions for an abnormal temperature and an overcurrent in order to protect itself. Thus, the semiconductor device is configured to minimize the risk of erroneous ignition.
In a case where the signal level of the output signal output from the drive circuit is the Low level, the electric potential of the power semiconductor ground (hereinafter also referred to as “power GND”), to which the power semiconductor device included in the ignition system as the semiconductor device is connected, may decrease to about −60 V due to the application of, for example, surge. The wiring extended to the battery ground (hereinafter also referred to as “battery GND”) to which the battery is connected affects such decrease.
When the electric potential of the power GND decreases to about −60 V while the signal level of the output signal output from the drive circuit is the Low level, the semiconductor device may malfunction, causing the switching element to perform the ON operation again. To prevent this, the semiconductor device has, as the protection function, the function of preventing unnecessary ignition operation such that the semiconductor device does not operate in response to a short-pulse signal such as a surge.